Towed active sonars comprise an emission antenna integrated into a submersible object also referred to as a towfish, and a receive antenna, for example a linear array, also referred to as a streamer. When the sonar is being used in dependent towing, the towfish and the streamer are connected in succession to one and the same towing cable. FIG. 1 illustrates a known configuration of an active sonar in dependent towing. The deck of the ship 10 is equipped with a towing device comprising a motorized winch 11 capable, via a fairlead 15, of towing a cable 12 to which are connected on the one hand a towfish 13 and on the other hand a streamer 14.
During a sonar mission, the streamer and then the towfish are first of all launched into the water by the deck crew. The towfish and the streamer are then towed by the ship at the desired depth of immersion, determined by the length of the cable and the speed of the ship. The emission and receive antennas are generally controlled from the ship, via electrical and/or optical information transmitted via conductors integrated into the towing cable. At the end of the mission, the towfish and the streamer are brought back up and placed on the deck.
In the known way, the emission antenna emits, by means of one or more transducers, an acoustic wave directed toward the seabed or into the column of water. The wave reflected off the seabed or off an object situated in the column of water is then detected by the acoustic receiver. A signal processing device operating on the signal received then allows the seabed or the detected object to be imaged. A precise measurement requires that the towed elements achieve good stability, particularly the acoustic antenna, so as to allow stable angular coverage. For that, the towfish is generally made of a submersible body suspended from the towing cable by means of fixing arms, which under the effect of gravity are able to generate a moment that tends to keep the towfish plumb.
This return moment generated by gravity may sometimes prove insufficient to ensure good stability of the towed object. Specifically, the flow of water around the object generates hydrodynamic forces the intensity of which increases with the speed of towing. These hydrodynamic forces may generate a rolling moment about the transport axis of the object, a pitching moment or even a yawing moment and unsettle the towed object by acting against the gravity return moment.
These disrupting hydrodynamic forces are particularly significant in instances in which the towed object has a shape that is asymmetric about one of these axes. The applicant company's patent document published under the reference FR2982579 and describing an articulated fairlead intended to optimize the operations of launching and retrieving the towfish is thus known. Such a towing device offers numerous advantages: it guarantees a minimum bend radius for the towing cable and makes it easier for the towed body to pass through the fairlead, making it possible to dispense with one articulated arm for grabbing hold of the towed body when raising the cable back up. The use of this articulated fairlead does, however, require the fixing arm to pass laterally. For this reason, the towed body has an architecture that is asymmetric. This physical asymmetry generates an asymmetry in the lift which may, for high towing speeds, notably for speeds corresponding to a Froude number (V/root (g*L), where V is speed in m/s, g is the constant due to gravity, L is the length of the body) at least equal to 1.5 and potentially exceeding 3, cause the towed body to rotate about the towing cable. At low speed, gravity is enough to stabilize the object.
Attempts are therefore being made to improve the stability of a submersible body the towing means of which exhibit asymmetry. More generally, it remains desirable to have available means for stabilizing the orientation in roll, pitch and yaw, of a body towed in a fluid by a cable.